Binary Selenides Research Articles

Preparation and Characterization of Copper Gallium Diselenide Thin Films from the Solgel-Derived Precursors

A simple process for preparing CuGaSe2 (CGS) absorber layers was developed in this study. The solgel-derived Cu-Ga-O precursor paste with variable Ga3+/Cu2+ ratios was coated on glass substrates using a doctor-blade technique. The precursor films were selenided with a selenium vapor at the temperature ranging from 250 to 550°C. The GIXRD patterns showed that single-phase CuGaSe2 through the whole films was obtained at a Ga3+/Cu2+ molar ratio of 1.5 on selenization at 450°C. The Raman measurements also indicated that the grown CuGaSe2 thin films exhibited the chalcopyrite structure. The SEM images of the films reveal that with an increase in Ga/Cu ratio in the films, the amount of Cu2Se particles on the surface of the film was reduced. The resistivity of the films was increased with the increase in Ga content in the films. The formation mechanism of CuGaSe2 thin films was proposed based on the XRD and Raman measurements of the films. The binary copper selenides are formed first, and then these phases lead to the formation of CuGaSe2.

Investigation of copper indium gallium selenide material growth by selenization of metallic precursors

We report a study of copper indium gallium selenide (CIGS) thin film growth in the annealing process at temperature range from 120°C to 600°C. Thin films were prepared by sputtering metal precursors and subsequent selenization process. Surface morphologies of thin films were observed by using high resolution field emission scanning electron microscopy (FESEM). Phases in quaternary systems Cu–In–Ga–Se were investigated by X-ray diffraction (XRD). Evolution of crystalline structure in the film surface was studied by Raman spectra. A possible reaction path from metallic precursors to a single CIGS phase was obtained by merging all results of SEM, XRD and Raman. Above 210°C, selenium reacted with Cu and In to form binary selenide. CuSe crystalline platelets were observed clearly in the film surfaces. When temperature was reaching 380°C, Cu2−xSe and InSe reacted with excess Se to form CuInSe2 (CIS) and contributed to the grain growth. Above 410°C, Ga-rich phase was detected in the films. With increased temperature, Ga diffused into CIS crystalline lattices. Finally, at 600°C, a single phase of Cu‐In‐Ga‐Se quaternary system was formed. A large number of triangular and hexagonal structures were observed in the film due to a re-crystalline process at a high annealing temperature.

On the Kinetics Description of Below-T<sub>g</sub> Structural Relaxation in Network Glass Formers

Phenomenological description of below-Tg structural relaxation known as long-term natural physical ageing in network glass formers exemplified by chalcogenide glasses of binary arsenic selenides was adequately developed within formalism of first-order kinetics. This kinetics was shown to obey a character stretched exponential behavior being treated as sequence of aligning-shrinkage stages throughout glassy matrix.

Characterization of Cu2ZnSnSe4 Thin Films Selenized with Cu2-xSe/ SnSe2/ZnSe and Cu/SnSe2/ZnSe Stacks

material is receiving an increased amount of attention for solar cell applications as an absorber layer because it consists of inexpensive and abundant materials (Zn and Sn) instead of the expensive and rare materials (In and Ga) in solar cells. We were able to achieve a cell conversion efficiency to 4.7% by the selenization of a stacked metal precursor with the Cu/(Zn + Sn)/Mo/glass structure. However, the selenization of the metal precursor results in large voids at the absorber/Mo interface because metals diffuse out through the top CZTSe layer. To avoid the voids at the absorber/Mo interface, binary selenide compounds of ZnSe and were employed as a precursor instead of Zn and Sn metals. It was found that the precursor with Cu//ZnSe stack provided a uniform film with larger grains compared to that with /ZnSe stack. Also, voids were not observed at the /Mo interface. A severe loss of Sn was observed after a high-temperature annealing process, suggesting that selenization in this case should be performed in a closed system with a uniform temperature in a environment. However, in the experiments, Cu top-layer stack had more of an effect on reducing Sn loss compared to top-layer stack.

ChemInform Abstract: Thermochemistry of Organoselenium and Organotellurium Compounds

AbstractReview: 152 refs.

Influences of plasma activation Se source on selenization of electrodeposited Cu-In-Ga metallic precursors

In this paper, the electrodeposited Cu-In-Ga metallic precursors have been selenized by using plasma activation Se source. The power of plasma has great influence on the grain growth of Cu(In1-xGax)Se2(CIGS). The films were shown to be single phase Cu(In0.7Ga0.3)Se2 when the plasma power was 75W. And the fact that high activity Se promotes the generation of binary selenide phase at a low temperature, thus helping the growth of single phase Cu(In0.7Ga0.3)Se2, was proved by XRD analysis of the films selenized at defferent temperatures and the comparison with the films prepared by ordinary selenization. Solar cells have been prepared and found that the single phase have no influence on battery performance. The efficiency can reach 9.4% by process optimization.

Single-step electrodeposition of a microcrystalline Cu2ZnSnSe4 thin film with a kesterite structure

Single-step electrodeposition synthesis of a Cu2ZnSnSe4 (CZTSe) film on a Mo-coated glass substrate from an acidic electrolyte containing Cu(II), Zn(II), Sn(IV), and Se(IV) species was investigated. The desired CZTSe film as the main phase was obtained at some selected applied potential ranges through reaction among binary selenides, Cu2Se, ZnSe and SnSe2, which were continuously formed in the present electrolyte containing all of the elements. Sulfurization of the as-deposited film at several temperatures under H2S gas flow resulted in the formation of corresponding mixed compounds of CZTSe and Cu2ZnSnS4 (CZTS), i.e., Cu2ZnSn(S,Se)4 (CZTSSe): specifically, sulfurization at temperatures higher than 500°C resulted in the formation of single-phase CZTSSe with S-rich compositions. By analyzing linear sweep voltammograms (LSVs) of sulfurized films under chopped irradiation, the films were confirmed to have p-type photoresponses; the film obtained by 500°C sulfurization showed the largest photoresponse because of its sufficiently large grain size and less voids, whereas the presence of an anodic spike in the LSV curve as well as the observation of a broad external quantum efficiency (EQE) spectrum suggested the requirement of further improvement in film quality for photovoltaic application.

Highly dense and crystalline CuInSe2 thin films prepared by single bath electrochemical deposition

Chalcopyrite CIS or CIGS have been regarded as promising absorbing materials for thin-film solar cells with widespread commercialization prospects. The most critical material properties of a CIS absorption layer that affect the overall PV performance include its microstructure and composition at a given bandgap energy. In this study, dense CISe films with high crystallinity and uniform, flat surfaces were fabricated on In2Se3/ITO employing single bath electrochemical deposition by adjusting the deposition parameters, such as the precursor concentration, pH, and applied potential. A simple formula is presented based on Faraday's law to quantitatively estimate the density of the electrodeposited thin films; from this, it was found that the as-deposited films had a very high relative density of 0.73. The high green density of the as-deposited film led to the full densification of the CISe film with ca. 10μm sized grains. The binary selenide phase remaining in the sintered film was subsequently etched out using a KCN solution, resulting in an overall Cu-deficient composition in the film of [Cu]/[In]=0.95.

Selenides for nonlinear optical applications

The authors have developed several binary, ternary, and quaternary selenide crystals for nonlinear optical (NLO) applications. The authors present an overview of the crystal growth and characterization of bulk and quasi-phase matched (QPM) crystals for their applications in mid-wave infrared (MWIR) and long-wave infrared (LWIR) regions. The authors have summarized the crystal growth, fabrication, and performance of gallium selenide and ternary and quaternary selenide compounds, silver gallium selenide, thallium arsenic selenide, and silver gallium germanium selenide crystals. Large crystals are grown by Bridgman method from the melt. These crystals transmit up to 16 μm wavelength, have high value of nonlinear conversion merit (d2/n3, d is the NLO coefficient, n is the refractive index) and have the lowest absorption coefficient compared to arsenides, phosphides, and other NLO materials. In addition to bulk crystals, the authors have developed ZnSe-based QPM materials for the MWIR and LWIR regions. The authors present their recent progress in the development of optical parametric ZnSe (OPZnSe) devices. Transmission measurements for these QPM film of thickness exceeding 1 mm show low-optical loss from 1 to 12 μm wavelength region. Nonlinear frequency conversion experiments were performed, demonstrating the vast spectral range of this material. Near-infrared wavelength conversion at 0.8 μm was obtained by frequency doubling a 1.6 μm wavelength beam with these devices.

Preparation of CuInSe2 thin films by selenization of co-sputtered Cu-In precursors using rapid thermal processing

CuInSe2 thin films have been synthesized by selenization of co-sputtered Cu-In precursors using rapid thermal processing (RTP). Heat treatments from 400 to 450 °C for periods between 1 min and 10 min were carried out on (Cu-In)/Se precursors. Phase evolution as function of reaction temperature and holding time was analyzed according to XRD and SEM results. Severe Se loss during RTP was proved in our experiments and has been reported by many other researchers. To solve the problem, a new effective way of reducing Se loss was presented, which is based on low temperature heat treatment at 250 °C before high temperature annealing. Nearly single-phase CuInSe2 thin films have been achieved by annealing precursors at 250 °C for 5 min then 450 °C for 1 min. Se loss can be significantly reduced via low temperature heat treatment by the fact that under 250 °C, Se is evaporated mildly and largely consumed as Cu-Se and In-Se binary selenides.

Characteristics of Cu(In,Ga)Se2 Films Prepared by Atmospheric Pressure Selenization of Cu-In-Ga Precursors Using Ditert-Butylselenide as Se Source

In this study, copper indium gallium diselenide [Cu(In,Ga)Se2; CIGS] films were prepared by selenization of Cu-In-Ga metallic precursors using ditert-butylselenide (DTBSe) under atmospheric pressure. Based on the results of θ-to-2θ X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), it was found that the films selenized at 300 or 400°C for 60 min showed the presence of Kirkendall voids along with the XRD signitures of pure copper (Cu) and certain intermediate binary selenides, copper indium diselenide (CIS) and CIGS depending on temperature while only a single CIGS structure was detected in those films selenized at 500 or 600°C for 60 min. A 5-temperature selenization process was found to enable the formation of CIGS structure with better crystalline quality and thickness uniformity. It is believed that intermediate binary and ternary selenides are formed sequentially with increasing completeness during low-temperature selenization stages of the 5-temperature selenization process. This enhances the subsequent formation of CIGS structure at high-temperature selenization stages of the 5-temperature selenization process with improved structural and morphological properties.

Crystallization of Cu 2ZnSnSe 4 compound by solid state reaction using elemental powders

Commercially available elemental Cu, Zn, Sn and Se powders were used to crystallize the stannite-type Cu 2ZnSnSe 4 compound through a solid state reaction. The Cu 2ZnSnSe 4 reaction chemistry was also modeled based on the thermal analysis and X-ray powder diffraction results. Se preferably reacted with Cu to form the α-CuSe and CuSe 2 phases at low reaction temperatures. The formation of the Cu 5Zn 8 intermetallic phase occurred through an intermediate reaction path for the ZnSe formation. At 320 °C, all of the elemental powders completely reacted to form the binary selenides of CuSe, ZnSe and SnSe. The crystallization of Cu 2ZnSnSe 4 began at 300 °C. The compositional investigation of the crystallized Cu 2ZnSnSe 4 compound revealed that the Se loss could not perfectly be avoided during the reaction. The crystallization of Cu 2ZnSnSe 4 occurred through the reaction between Cu 2SnSe 3 and ZnSe.

In situ observation of phase formation in a Cu–In–2Se precursor layer using micro-Raman scattering spectroscopy

In situ observation of phase formations in Cu–In–2Se (atomic ratio) precursor layers was carried out using temperature-dependent micro-Raman scattering spectroscopy. Without the protection of a high Se vapor pressure, intermediate In–Se and Cu–Se selenides and CuAu-ordered CuInSe2 (CIS) were easily formed in the amorphous precursor layer, which have atomic structures arranged in a way close to that of α-CIS. The binary selenides were transformed to the α-CIS by in-/out-diffusion of the mobile Cu and Se atoms/ions. The γ-CuSe and α-CIS began to form at a temperature as low as ∼170 °C.

Cu 2ZnSnSe 4 films by selenization of Sn–Zn–Cu sequential films

This paper deals with the formation of Cu 2ZnSnSe 4 (CZTS) in the process of selenization of metal precursor layers in elemental selenium vapour. Metallic precursors were sequentially evaported from Sn, Zn and Cu sources. Precursor Sn–Zn–Cu films have a “mesa-like” structure and consist mainly of Cu 5Zn 8 and Cu 6Sn 5 phases. It was confirmed that the formation of different binary copper selenides is the dominating process of selenization in elemental Se vapour at temperatures up to 300 °C. The formation of kesterite CZTS films begins at 300 °C and dominates at higher temperatures, always resulting in multiphase films that consist of high-quality Cu 2ZnSnSe 4 crystals and of a separate phase of ZnSe.

Oxide-melt solution calorimetry of selenides: Enthalpy of formation of zinc, cadmium, and lead selenide

Selenium is an important trace element. Its geochemical cycles involve various oxidation states, including Se2−, yet thermodynamic data for selenides are sparse. A general method for selenide thermochemistry has been developed by using oxidative drop-solution calorimetry in a molten oxide solvent. The samples are dropped from room temperature into molten sodium molybdate (3Na2O·4MoO3) solvent at 975 K, with oxygen bubbling through the melt to ensure rapid and complete conversion of selenide to dissolved selenate species. This method is analogous to that developed for sulfides (Deore and Navrotsky 2006). Complete dissolution of selenides is documented by furnace tests and visual inspection, and supported by consistent results for enthalpy. Enthalpies of formation (Δf H °, kJ/mol) from the elements were determined for zinc selenide (ZnSe) (−169.94 ± 4.31 kJ/mol), cadmium selenide (CdSe) (−139.04 ± 6.13 kJ/mol), and lead selenide (PbSe) (−99.26 ± 6.06 kJ/mol). These three Δf H ° values agree with previously published data, often obtained by less direct means. This is the first report of the enthalpy of formation of CdSe measured by any calorimetric method. For ZnSe, a more reliable Δf H ° value is proposed since prior data are quite scattered. The results confirm that oxidative drop-solution calorimetry is an available method for selenide thermochemistry. It should be applicable to binary and multinary selenides with large homogeneity ranges, to solid solutions, and to nanophase materials.

Structural, optical and microscopic properties of chemically deposited In2Se3 thin films

Solar cell technologically important binary indium selenide thin film has been developed by relatively simple chemical method. The reaction between indium chloride, tartaric acid, hydrazine hydrate and sodium selenosulphate in an aqueous alkaline medium at room temperature gives deposits In2Se3 thin film. Various preparative parameters are discussed. The as grown films were found to be transparent, uniform, well adherent, red in color. The prepared films were studied using X-ray diffraction, scanning electron microscopy, atomic absorption spectroscopy, Energy dispersive atomic X-ray diffraction, optical absorption and electrical conductivity properties. The direct optical band gap value Eg for the films was found to be as the order of 2.35 eV at room temperature and having specific electrical conductivity of the order of 10−2 (Ω cm)−1 showing n-type conduction mechanism. The utility of the adapted technique is discussed from the point of view of applications considering the optoelectric and structural data obtained.

Synthesis of Cu(In<sub>0.75</sub>Al<sub>0.25</sub>)Se<sub>2</sub> Thin Films from Binary Selenides Powder Compacted Targets by Sputtering and Selenization

Cu(In0.75Al0.25)Se2 thin films were prepared using both one-step sputtering and two-stage process involving sputtering and selenization. Structure and properties of the films were analyzed and compared between the methods. All films had good adhesion to the substrate and showed strong (112), (220)/(204) peaks while minor secondary phase CuxSe was observed in the films from selenization. This paper compares the visibility of one step sputtering process and selenization of sputtered binary selenides layers to produce Cu(InAl)Se2 thin films (CIAS) for solar cell absorber applications. One step sputtering deposition from binary selenides powder compacted targets can produces CIAS films with better structural and optical properties compared to films produced by selenization of sputtered precursors.

The formation of CuInSe 2 thin film solar cell absorbers from electroplated precursors with varying selenium content

We present results from real-time X-ray diffraction experiments on the formation of CuInSe 2 solar cell absorbers by annealing precursors, produced by simultaneous electrodeposition of copper, indium and selenium. The investigations reveal, that a reduced amount of electrochemically deposited selenium is the decisive parameter in order to realise a chalcopyrite formation behaviour as observed for sputtered stacked elemental layer (SEL) precursors. A simultaneous electrodeposition of the elements copper, indium and selenium in the molar ratio 1:1:2 of the chalcopyrite CuInSe 2 leads to the formation of binary copper and indium selenides during the electrodeposition process. The existence of binary selenides besides the intermetallic phase Cu 11In 9 as initial phases leads to an unfavourable absorber morphology. This can be explained by the observed semiconductor formation mechanism. A reduction of the deposited amount of selenium favours the formation of the intermetallic compound Cu 11In 9 and reduces the amount of binary selenides. These precursors show a formation behaviour and resulting absorber morphology as known for sputtered SEL precursors.

Predicted formation reactions for the solid-state syntheses of the semiconductor materials Cu 2SnX 3 and Cu 2ZnSnX 4 (X = S, Se) starting from binary chalcogenides

The compounds Cu 2ZnSnX 4 and Cu 2SnX 3 (X = S or Se) are promising semiconductor materials for thin film photovoltaic applications. Based on a crystallographic growth model we derive the solid-state reactions for these four compounds starting from the binary sulphides and selenides of copper, zinc and tin. Exploiting these predicted solid-state reactions which are promoted by epitaxial relations between the educts will result in fast formation reactions as preferred in technical processes. The direct formation of Cu 2ZnSnX 4 is concurring with a two-step process in which Cu 2SnX 3 occurs as an intermediate product.

Real-time investigations on the formation of CuInSe 2 thin film solar cell absorbers from electrodeposited precursors

In this article, we present results of a detailed real-time X-ray diffraction (XRD) study on the formation of CuInSe 2 from electroplated precursors. The solid-state reactions observed during the selenisation of three different types of precursors are presented. The first type of precursors (I) consists of the nanocrystalline phases Cu 2− x Se and InSe at room temperature, which react to CuInSe 2 starting at 470 K. The second type of precursor (II) shows an inhibited CuInSe 2 formation out of the initial phases Cu 2− x Se and γ-In 2Se 3 starting at 400 K. The third precursor type (III) shows completely different selenisation behaviour. Starting from the intermetallic compound Cu 11In 9 and amorphous selenium, the formation of the binary selenides In 4Se 3 and CuSe is observed after the melting point of selenium at 494 K. After selenium transfer reactions, the compound semiconductor CuInSe 2 is formed out of Cu 2− x Se and InSe. This type (III) reaction path is well known for the selenisation of SEL precursors (stacked elemental layers of sputtered copper and indium and thermally evaporated selenium).